Within the growing realm of UV curing technology, one of the most important applications is in photo-imaging. Ever since the birth of photography, new and innovative ways of obtaining images by exposure to light have been explored; even the silver halide process itself, which still forms the core of non-digital photography, has undergone substantial change, as for example when the introduction of T-grain emulsions took place.
Although substantial improvements in photopolymer technology have been made over the last 20 years, the sensitivity of the processes is still very limited compared with the photosensitivity of the silver halide process. One of the major goals in photopolymer science is to approach the sensitivity of silver halide based processes.
Two basic methods of increasing the photopolymer quantum yield beyond unity exist. The first of these is most familiar as the acrylate chemistry used in most commercial free radical UV cure systems. The approach here is that of the chain reaction in which one or more photoinitiators that are exposed to electromagnetic radiation of a suitable wavelength/energy, absorb photons incident upon them. The energy of the photon is used chemically by the photoinitiators to generate free radicals in the irradiated substrate, each of which is then capable of causing many polymerisable molecules to polymerise very quickly resulting in a high quantum yield of polymerisation. Thus the quantum yield for this process is high but still not as high as that overall for silver halide photography.
The second fundamental form of photopolymer quantum yield enhancement is exemplified by the cationic UV curing systems. In this instance, the absorbed photon generates a catalytic monomer species which is capable of catalysing polymerisation, cross-linking, or even molecular cleavage. This technology has been described as capable of producing “living polymers” which will continue growing as long as substrate monomer molecules are still available. The reactions are, however, relatively slow compared with the chain reactions of the free radical process. Furthermore, although the quantum yield in terms of reacted molecules is theoretically near infinite, the slow reactions limit spatial resolution by reason of diffusion of active species out of the imaged area.
A limitation in terms of photopolymer imaging has always been the amount of time needed to deliver an adequate amount of energy to the area to be imaged. The delivery of a large amount of energy is easy. High intensity sources of radiation, simple reflectors and conveyor belts used in combination enable this aim to be achieved. For imaging, the radiation needs to be collimated and delivered in a controlled fashion. To collimate the output from any lamp involves a substantial loss of intensity. The subsequent use of optical components and even phototools serves to reduce the energy from even a very powerful source to a remarkably low level.
It is within this environment that the usage of lasers for imaging has developed. Although the radiant flux that such lasers will deliver is relatively low, the intrinsic collimation and the intensity of photons delivered at a given wavelength, make the laser a useful light source. Computer guided beam manipulation in combination with mirrors enables one to eliminate photo tool usage, and further enhances the number of photons available for the photochemistry. Nevertheless, these improvements have been only incremental, and laser imaged photopolymer processes are still slow.
In the silver halide process, the actual efficiency of the photochemistry is relatively low compared with chain reaction processes. Each photon produces only a single silver atom, thus the quantum efficiency is only one (or, in practice, less than one). The overall sensitivity of the silver process to light only becomes obvious by virtue of the development step when many more silver atoms are produced. Wherever a silver atom has been produced by irradiation with light, the silver available accelerates the development reaction, via an autocatalytic reaction, which in turn produces more silver. Thus the quantum efficiency of “image formation” can be varied from one to infinity, owing to the propagation that occurs in this second stage. However, the propagation occurs only within a grain boundary, viz. the silver atom produced in one grain can be completely developed but adjacent grains are not developed. Resolution of image details is, therefore, limited by grain size in the silver halide process.
Bradley et al Journal of Photochemistry and Photobiology A: chemistry 100 (1996) 109-118 describes the development of vinyl dioxolane based monomers as a more amenable alternative to vinyl ethers conventionally used as monomers for cationic UV curing. Such a material is (2,2′-diphenyl-4-methylene-1,3-dioxolane).

EP-A-1307783 describes a process in which a protected (also referred to as “blocked” or “latent”) photoinitiator is included in a reactive substrate. The protected photoinitiator is deprotected in situ and is available for a subsequent photoinitiated reaction. The protected photoinitiator is a protected ketone photoinitiator in which the ketone group is protected by a methylene 1, 3 dioxolane group.
It has been found that the prior art ketone photoinitiators having a methylene-1,3-dioxolane moiety are not completely stable and decompose at room temperature or in the presence of light or very small quantities of even quite weak acid. The manufacture, handling and storage of photoinitiators based on such materials in commercial quantities may thus be difficult. A need exists for a more stable material which would still function as a blocked initiator under the correct conditions.